1. Field of the Disclosure
This disclosure relates to acoustic sensors having high acoustic attenuation backings, methods of making such sensors and using same in acoustic tools for downhole well logging and other applications.
2. Description of Prior Art
Typical downhole acoustic and ultra acoustic logging tools utilize one or more acoustic transducers to generate acoustic signals in a wellbore (or borehole). The frequency of a typical transducer is in the ultrasonic range, generally 100 kHz or more. Acoustic frequencies in this range are used to achieve better resolution in the confined space of a wellbore. Acoustic transducers often utilize piezoelectric active elements in ceramic or metallic enclosures. Such transducers often transmit acoustic signals through the wellbore fluid into the earth formation surrounding the borehole. Various acoustic receivers in the tool sense acoustic signals reflected from the formation. Such received signals are processed to determine a property of interest of the formation and/or the borehole wall or of the wellbore fluid, itself. A backing is typically placed on the back side of the transducer to attenuate acoustic signals impinging on such side to reduce the ringing effect and to reduce reflections from the back side of the transducer so that a clean signal, consisting of only acoustic waves that emanate from the front of the transducer, are launched into the medium with which it is in contact. The main acoustic design criteria for a backing material are: (i) to minimize acoustic reflection at the backing-transducer interface by matching, as closely as possible, the acoustic impedance of the backing material to the acoustic impedance of the transducer and (ii) to minimize the return of any acoustic waves to that interface once they have entered the backing material through the use of acoustic scattering to many other directions or through the use of acoustic absorption. Typically, acoustic transducer backing materials are constructed of a solid matrix, usually an epoxy or another polymer filled with solid particle fillers such as tungsten, micro balloons, etc., that act as acoustic scatterers to different directions to reduce the return of acoustic waves to the backing-transducer interface. Such backings often do not effectively attenuate all or substantially all the compressional waves received at the interface between the transducer and the backing.
The disclosure herein provides transducers with backings that improve attenuation or attenuate substantially all compressional waves that are transmitted into the backing from the back side of the transducer.